1. Field of the Invention
This invention relates to steam generating systems in general and more specifically to a system for automatically injecting a cool water mist into a steam bound boiler feed pump.
2. Background of the Invention
In a typical steam generator, a hot water boiler is fed by a centrifugal boiler feed pump. For a variety of reasons it is not unusual for such pumps to become "steam bound" and when this occurs, the efficiency of the boiler pump drops considerably. During the course of disclosure, the term steam bound will be used interchangeably with the term "air bound" as those two terms are understood to imply essentially the same undesirable condition. To overcome this condition, the steam plant operator often has to cool down the steam bound pump by applying cold water to its exterior housing. Several other approaches are discussed in the prior art. For example, Vogler U.S. Pat. No. 3,126,875 discloses injecting cold water into the intake side of a water pump in order to eliminate vapor lock therein. The cold water is injected into the pump in response to a pressure differential measuring device which monitors the pressure in a hot water tank. Similarly, British Pat. No. 343,385 discloses injecting a second liquid into a centrifugal pump and calls for the liquid to be under pressure and to enter the pump through a plurality of nozzels located in the housing. Other prior art references of relevance include: Hariveau U.S. Pat. No. 1,581,204; Hutton U.S. Pat. No. 3,286,639; Jackson U.S. Pat. No. 3,504,986; German Pat. Nos. 304,763; 475,711; and 556,579; and British Pat. No. 308,442.
Unfortunately, the techniques disclosed by the prior art are often complicated, both in structure and function. Since the problem of steam bound boiler feed pumps is common, a direct and straight-forward solution to this problem was sought. Additionally, the problem of steam bound pumps is frequently associated with low water boiler failures. For example, a total of 44 accidents occurred due to low water failures in power boilers during the period of Jan. 1, 1973 through Dec. 31, 1973 as reported by the National Board of Boiler and Pressure Vessel Inspectors. Those accidents resulted in four known injuries. Similarly, low water failures resulted in 78 accidents in heating boilers during the same period. These statistics only reflect reported insurance accidents and do not take into account factors such as lost production time and boiler damage repair cost. For example, if the boiler tubes burn out they must be replaced and the plant must be shut down during the replacement period. While not all low water failures are attributable to malfunctioning feed pumps, nevertheless, malfunctioning pumps are believed to be a significant factor in such difficulties. Therefore a means was sought which would prevent low water boiler failures due to steam bound pumps. This, of course, would increase the safety of steam generators and generally improve reliability. The factor of reliability is very important, since it is often necessary to have a complete steam generator system in standby condition ready for operation if the use of steam is critical. For example, a naval man of war in the attack condition; a petroleum oil cracking tower or a hospital must be able to maintain steam power under all conditions. By automatically eliminating the steam bound pump condition, it may be possible to eliminate the necessity of a secondary system in such steam generating operations. Therefore, from the safety and reliability point of view, the ability to automatically clear a steam bound boiler pump is a very important capability. It is believed that the present invention has applicability to a wide range of prior art steam generators, including stationary, marine, locomotive and portable boilers. Also, while a centrifugal pump is used as an illustrative example, it is to be understood that this system may be applied to many other pumps known in the prior art. It was in the context of the foregoing prior art that a simple, safe, efficient and reliable means was sought to solve the problem of steam bound boiler feed pumps.
Another problem frequently associated with steam bound or air bound pumps is the difficulty of keeping the packing seals lubricated when the pump goes into a steam bound condition. Many prior art pumps are equipped with liquid lubricated packing seals which require a continual application of water in order to keep them adequately lubricated. In the steam bound condition these seals will frequently dry out and burn up. When a seal burns up it may require that the boiler system be turned off until such time as the liquid lubricated packing seals can be replaced.
Several different types of packing seals are known to those of ordinary skill in the art. Some are of the passive variety and others include features for improving the circulation of the lubricating liquid. An example of a circulating seal is the Dura circulating seal, Type P.T., manufactured by the Dura Metallic Corporation of Kalamazoo, Mich. One of the advantages of using such circulating seals is that it reduces the possibility of steam flashing, and thereby protects the seal from running "dry".
One technique to prevent lubricating seals from running dry is to provide a special bypass channel from the high side of a pump to the packing seals. A controllable valve is often located in that channel. Therefore, the amount of lubrication to the seal can be controlled. This approach is disclosed in a publication entitled "Pump Maintenance" published by the Allis-Chalmers Manufacturing Company of Milwaukee, Wisc. During the course of operation, it is generally necessary for a boiler maintenance engineer to provide lubrication to a "steam bound pump" as soon as possible so as to avoid the problem of burning the packing seals. The present invention includes a method of insuring that such packing seals do not run dry and therefore it helps to overcome the prior art problem of having the packing seals burn when the pump is in the steam bound condition.